repack-promisor: allow excluding objects from the rebuilt promisor pack

Add a to_drop oidset parameter to repack_promisor_objects(). When it is
non-NULL, write_oid() omits those objects from the rebuilt promisor
pack. This is the mechanism --drop-filtered will use to remove promisor
blobs, i.e. rebuild the promisor pack without them.

All existing callers pass NULL, so behavior is unchanged.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Siddharth Shrimali <r.siddharth.shrimali@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/repack.c

@@ -430,7 +430,7 @@ int cmd_repack(int argc,
 		strvec_push(&cmd.args, "--delta-islands");
 
 	if (pack_everything & ALL_INTO_ONE) {
-		repack_promisor_objects(repo, &po_args, &names, packtmp);
+		repack_promisor_objects(repo, &po_args, &names, packtmp, NULL);
 
 		if (existing_packs_has_non_kept(&existing) &&
 		    delete_redundant &&

repack-promisor.c

@@ -6,10 +6,12 @@
 #include "path.h"
 #include "repository.h"
 #include "run-command.h"
+#include "oidset.h"
 
 struct write_oid_context {
 	struct child_process *cmd;
 	const struct git_hash_algo *algop;
+	const struct oidset *to_drop;
 };
 
 /*
@@ -23,6 +25,15 @@ static int write_oid(const struct object_id *oid,
 	struct write_oid_context *ctx = data;
 	struct child_process *cmd = ctx->cmd;
 
+	/*
+	 * Objects in to_drop are being removed from the repository, so
+	 * omit them from the rebuilt promisor pack. Each such object is a
+	 * promisor object and therefore remains recoverable from the
+	 * promisor remote.
+	 */
+	if (ctx->to_drop && oidset_contains(ctx->to_drop, oid))
+		return 0;
+
 	if (cmd->in == -1) {
 		if (start_command(cmd))
 			die(_("could not start pack-objects to repack promisor objects"));
@@ -81,7 +92,8 @@ static void finish_repacking_promisor_objects(struct repository *repo,
 
 void repack_promisor_objects(struct repository *repo,
 			     const struct pack_objects_args *args,
-			     struct string_list *names, const char *packtmp)
+			     struct string_list *names, const char *packtmp,
+			     const struct oidset *to_drop)
 {
 	struct write_oid_context ctx;
 	struct child_process cmd = CHILD_PROCESS_INIT;
@@ -98,6 +110,7 @@ void repack_promisor_objects(struct repository *repo,
 	 */
 	ctx.cmd = &cmd;
 	ctx.algop = repo->hash_algo;
+	ctx.to_drop = to_drop;
 	odb_for_each_object(repo->objects, NULL, write_oid, &ctx,
 			    ODB_FOR_EACH_OBJECT_PROMISOR_ONLY);
 

repack.h

@@ -3,6 +3,7 @@
 
 #include "list-objects-filter-options.h"
 #include "string-list.h"
+#include "oidset.h"
 
 struct pack_objects_args {
 	char *window;
@@ -100,7 +101,8 @@ void generated_pack_install(struct generated_pack *pack, const char *name,
 
 void repack_promisor_objects(struct repository *repo,
 			     const struct pack_objects_args *args,
-			     struct string_list *names, const char *packtmp);
+			     struct string_list *names, const char *packtmp,
+			     const struct oidset *to_drop);
 
 struct pack_geometry {
 	struct packed_git **pack;